Carbo-nitriding process for martensitic stainless steel and stainless steel article having improved corrosion resistance

ABSTRACT

A method for producing a case-hardened martensitic stainless steel article includes: providing an article comprised, at least in part, of a martensitic stainless steel, carburizing the article within a temperature range of 1625° F.-1680° F. (885° C.-916° C.), and then carbo-nitriding the article within a temperature range of 1575° F.-1625° F. (857° C.-885° C.). An article, such as a bearing ring, comprising such a case-hardened martensitic stainless steel is also disclosed.

CROSS-REFERENCE

This application claims priority to U.S. provisional patent applicationNo. 61/492,675, filed on Jun. 2, 2011, the contents of which are fullyincorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The Government of the United States of America may have rights in thisinvention as a result of contract numbers N68335-09-C-0227 andN68335-11-C-0395 awarded by the Department of Navy.

TECHNICAL FIELD

The present invention generally relates to a method for treatingmartensitic stainless steel, an article produced thereby and a bearingring.

DESCRIPTION OF RELATED ART

High performance mechanical systems, such as bearing and gears inadvanced gas turbine engines, are required to operate at ever increasingspeeds, temperatures and loads. Advanced high temperature case hardenedbearing steel, such as Pyrowear® 675, has been developed for theseapplications. The basic mechanical properties and desired microstructurecan be achieved by conventional carburizing techniques, but corrosionresistance is not substantially better than conventional bearing steelssuch as M50 and 440C.

Pyrowear® 675 is a stainless steel product of Carpenter Technology(www.cartech.com) and contains the following elements by wt %: 0.07 C,0.65 Mn, 0.40 Si, 13.00 Cr, 2.60 Ni, 1.80 Mo, 0.60 V, 5.40 Co, balanceFe.

Carbo-nitriding is a surface modification process utilized to increasethe surface hardness of metals by introducing both carbon and nitrogeninto the metal matrix (case). This process is accomplished by diffusionof the carbon and nitrogen at elevated temperatures according to one ofseveral known methods. For example, gas carbo-nitriding is one knownmethod that has been successfully demonstrated on many metals toincrease surface hardness of low carbon metals up to a surface hardnessof HRc 60-62 (697-745 HV).

Known techniques for carburizing and/or carbo-nitriding stainless steelalloys are disclosed in GB 2 238 953, U.S. Pat. Nos. 4,154,629,5,873,956, 7,186,304, 7,648,588, 6,179,933 and 5,873,956, and US PatentPublication No. 2002/0037120.

SUMMARY

However, known techniques for carburizing and/or carbo-nitridingstainless steel alloys have not been capable of achieving high corrosionresistance while maintaining satisfactory mechanical properties andmicrostructure.

It is therefore an object of the present teachings to disclose animproved thermal process for carbo-nitriding martensitic stainlesssteel, such as Pyrowear® 675, as well as improved articles containingsuch steel, such as bearing rings. Such articles preferably provideenhanced corrosion resistance while maintaining satisfactory mechanicalproperties and microstructure.

This object is achieved by one or more of the appended independentclaims. Further developments of the present teachings are recited in thedependent claims.

According to one aspect of the present teachings, a process and anarticle are disclosed, in which carbon and nitrogen are simultaneouslyintroduced into the surface (case) of the stainless steel (e.g.,Pyrowear® 675), preferably at a relatively low pressure vacuum. Thisheat treatment process comprises first carburizing the stainless steelto produce the desired hard case depth at a higher furnace (soaking)temperature, followed by a combined carburizing-nitriding cycle at alower furnace temperature.

Carburizing is preferably accomplished by introduction of a saturatingamount of carbon, such as acetylene or propane, into the atmosphere soas to coat or cover the surface of the metal, followed by a diffusionphase, in which the vacuum chamber is evacuated to a sub-atmospherepressure while maintaining the soaking temperature constant or at leastrelatively constant.

Subsequent to the carburizing cycle, nitrogen (e.g. ammonia or N₂) isthen introduced into the carburizing atmosphere in the form of a gas tocreate a top or outer metal layer (case) having both carbon and nitrogenpresent in the form of metal carbides and metal nitrides. Thecarbo-nitriding cycle is preferably performed in substantially the sameway, albeit preferably at a lower soaking temperature than in thecarburizing cycle.

The metal part is then preferably quenched and tempered to produce thedesired hardness, microstructure and retained austenite.

Further embodiments of the present teachings include, but are notlimited to:

1. A method comprising:

(a) placing an article into a vacuum chamber, the article beingcomprised of a martensitic stainless steel consisting essentially of0.02-0.50 wt % C, 0.1-1.5 wt % Mn, 0.10-2.0 wt % Si, 8.0-20.0 wt % Cr,1.0-3.5 wt % Ni, 0.40-3.0 wt % Mo, 0.40-2.0 wt % V, 1.0-10.0 wt % Co,the balance being Fe (and unavoidable impurities),

(b) vacuum carburizing the article within a temperature range of 1625°F.-1680° F. (885° C.-916° C.) by repeating a plurality of cyclescomprising (i) introducing acetylene and/or propane into the vacuumchamber and (ii) then evacuating the vacuum chamber to about 0.1 atm(about 10 kPa), and

(c) then vacuum carbo-nitriding the article within a temperature rangeof 1575° F.-1625° F. (857° C.-885° C.) by repeating a plurality ofcycles comprising (i) introducing acetylene and/or propane and ammoniainto the vacuum chamber and (ii) then evacuating the vacuum chamber toabout 0.1 atm (about 10 kPa).

2. A method comprising:

(a) providing an article comprised, at least in part, of a martensiticstainless steel,

(b) carburizing the article within temperature range of 1625° F.-1680°F. (885° C.-916° C.), and

(c) then carbo-nitriding the article within a temperature range of 1575°F.-1625° F. (857° C.-885° C.), thereby producing a case-hardenedmartensitic stainless steel article.

3. The method according to the above-mentioned embodiment 2, wherein thestainless steel in step (a) consists essentially of 0.02-0.50 wt % C,0.1-1.5 wt % Mn, 0.10-2.0 wt % Si, 8.0-20.0 wt % Cr, 1.0-3.5 wt % Ni,0.40-3.0 wt % Mo, 0.40-2.0 wt % V, 1.0-10.0 wt % Co, the balance beingFe (and unavoidable impurities).

4. The method according to any preceding above-mentioned embodiment,wherein stainless steel in step (a) consists essentially of 0.03-0.10 wt% C, 0.5-1.0 wt % Mn, 0.20-0.6 wt % Si, 11.0-15.0 wt % Cr, 2.40-3.0 wt %Ni, 1.50-2.0 wt % Mo, 0.40-0.80 wt % V, 5.0-6.0 wt % Co, the balancebeing Fe (and unavoidable impurities).

5. The method according to the above-mentioned embodiment 4, wherein thestainless steel in step (a) contains the following weight percentages ofelements:

Mo+V=0.8 to 4.0 wt % and

Co+Ni=2.0 to 12.0 wt %.

6. The method according to any preceding above-mentioned embodiment,wherein the stainless steel in step (a) consists of 0.07 C, 0.65 Mn,0.40 Si, 13.00 Cr, 2.60 Ni, 1.80 Mo, 0.60 V, 5.40 Co, the balance beingFe (e.g., Pyrowear® 675), or the stainless steel is CSS-42L of LatrobeSpecialty Steel (0.12 C, 14.00 Cr, 0.60 V, 2.00 Ni, 4.75 Mo, 12.5 Co,0.02 Cb, the balance being Fe).

7. The method according to any preceding above-mentioned embodiment,wherein the nitrogen:carbon (e.g., ammonia:acetylene orpropane/acetylene) differential ratio in step (c) is between about6-8:1.

8. The method according to any preceding above-mentioned embodiment,wherein the total time for steps (b) and (c) is between 15-18 hours.

9. The method according to any preceding above-mentioned embodiment,wherein the carbo-nitrided article possesses a hard carburized case upto a depth of 900-1125 μm (38-45 mil).

10. The method according to any preceding above-mentioned embodiment,wherein the carbo-nitrided article exhibits a presence of nitrogen(nitrides) at least up to depth of 250 μm (10 mil).

11. The method according to any preceding above-mentioned embodiment,wherein the carbo-nitrided article contains approximately 0.5 wt %nitrogen at the surface and approximately 0.2 wt % nitrogen at a depthof about 250 μm (10 mil).

12. The method according to any preceding above-mentioned embodiment,wherein the carbo-nitrided article contains carbon in the range of 1.5-2wt % in the hardened case, preferably at least up to a depth of 500 μm.

13. The method according to any preceding above-mentioned embodiment,wherein the carbo-nitrided article is subsequently:

(d) quenched in gaseous nitrogen and

(e) then tempered twice at about 575° F.-625° F. (301° C.-329° C.) forabout 2 hours.

14. The method according to any preceding above-mentioned embodiment,wherein the carbo-nitrided article has a Vickers hardness of about 840HV (65.0 HRc).

15. The method according to any preceding above-mentioned embodiment,wherein the case of the article has a hardness of at least 800 Hv (˜63.7HRc) at a depth between 0.010 to 0.039 in (0.25 to 1.0 mm).

16. The method according to any preceding above-mentioned embodiment,wherein the case of the article has a hardness of between 800-850 Hv(˜63.7 to 65.4 HRc) at a depth between 0.010 to 0.039 in (0.25 to 1.0mm).

17. The method according to any preceding above-mentioned embodiment,wherein the case of the article has a hardness of at least 63 HRc (˜772HV) at 149° C. (300° F.).

18. The method according to any preceding above-mentioned embodiment,wherein the case of the article has a hardness of at least 62 HRc (˜745HV) at 204° C. (400° F.).

19. The method according to any preceding above-mentioned embodiment,wherein the carbo-nitrided article contains at least 0.5 wt % nitrogenin at least one portion of the hardened case and at least 0.2 wt %nitrogen at a depth of about 200 μm (8 mil).

20. The method according to any preceding above-mentioned embodiment,wherein the carbo-nitrided article exhibits a residual stress of atleast 500 MPa in the hardened case.

21. The method according to any preceding above-mentioned embodiment,wherein at least one portion of the hardened case of the carbo-nitridedarticle comprises at least 16 volume percent of retained austenite.

22. The method according to any preceding above-mentioned embodiment,wherein the article is a bearing ring.

23. An article produced according to the method of any precedingabove-mentioned embodiment.

24. A bearing ring comprised of a carbo-nitrided martensitic stainlesssteel having:

a hard carburized case up to a depth of about 900-1125 μm (38-45 mil),

a presence of nitrogen or nitrides up to depth of 250 μm (10 mil), about0.3-0.7 wt %, preferably about 0.5 wt %, nitrogen at the surface andabout 0.1-0.3 wt %, preferably about 0.2 wt %, nitrogen at a depth ofabout 250 μm (10 mil), and

carbon in the range of 1.5-2 wt % in the hardened case.

25. The bearing ring according to the above-mentioned embodiment 24,wherein the bearing ring has a core consisting essentially of 0.02-0.50wt % C, 0.1-1.5 wt % Mn, 0.10-2.0 wt % Si, 8.0-20.0 wt % Cr, 1.0-3.5 wt% Ni, 0.40-3.0 wt % Mo, 0.40-2.0 wt % V, 1.0-10.0 wt % Co, the balancebeing Fe (and unavoidable impurities).

26. The bearing ring according to the above-mentioned embodiment 24,wherein the bearing ring has a core consisting essentially of 0.03-0.10wt % C, 0.5-1.0 wt % Mn, 0.20-0.6 wt % Si, 11.0-15.0 wt % Cr, 2.40-3.0wt % Ni, 1.50-2.0 wt % Mo, 0.40-0.80 wt % V, 5.0-6.0 wt % Co, thebalance being Fe (and unavoidable impurities).

27. The bearing ring according to the above-mentioned embodiments 24-26,wherein the bearing ring has a core containing the following weightpercentages of elements:

Mo+V=0.8 to 4.0 wt % and

Co+Ni=2.0 to 12.0 wt %.

28. The bearing ring according to the above-mentioned embodiments 24-27,wherein the bearing ring has a core consisting of 0.07 C, 0.65 Mn, 0.40Si, 13.00 Cr, 2.60 Ni, 1.80 Mo, 0.60 V, 5.40 Co, the balance being Fe(e.g., Pyrowear® 675), or 0.12 C, 14.00 Cr, 0.60 V, 2.00 Ni, 4.75 Mo,12.5 Co, 0.02 Cb, the balance being Fe (e.g., CSS-42L of LatrobeSpecialty Steel).

29. The bearing ring according to the above-mentioned embodiments 24-28,wherein the hardened case of the bearing ring has a hardness of at least800 Hv (˜63.7 HRc) at a depth between 0.010 to 0.039 in (0.25 to 1.0mm).

30. The bearing ring according to the above-mentioned embodiments 24-29,wherein the hardened case of the bearing ring has a hardness of between800-850 Hv (˜63.7 to 65.4 HRc) at a depth between 0.010 to 0.039 in(0.25 to 1.0 mm).

31. The bearing ring according to the above-mentioned embodiments 24-30,wherein the hardened case of the bearing ring has a hardness of at least63 HRc (˜772 HV) at 149° C. (300° F.).

32. The bearing ring according to the above-mentioned embodiments 24-31,wherein the hardened case of the bearing ring has a hardness of at least62 HRc (˜745 HV) at 204° C. (400° F.).

33. The bearing ring according to the above-mentioned embodiments 24-32,wherein the hardened case of the bearing ring exhibits a residual stressof at least 500 MPa.

34. The bearing ring according to the above-mentioned embodiments 24-33,wherein at least one portion of the hardened case of the bearing ringcomprises at least 16 volume percent of retained austenite.

Further objects, embodiments, advantages and designs will be explainedin the following with the assistance of the exemplary embodiments andthe appended Figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a hardness depth profile for carburized and carbo-nitridedPyrowear® 675 tempered at 600° F. (2 times for 2 hours).

FIG. 2 shows the hot hardness of carburized and carbo-nitrided Pyrowear®675 up to 600° F.

FIG. 3 shows a corrosion rate comparison between carburized andcarbo-nitrided Pyrowear® 675.

FIG. 4 shows XRD results for carburized and carbo-nitrided Pyrowear®675.

FIG. 5 shows an approximate atom wt % of carbon and nitrogen asdetermined by Auger Electron Spectroscopy (AES).

FIG. 6 shows residual stress profiles of carburized and carbo-nitridedPyrowear® 675.

FIG. 7 shows retained austenite profiles of carburized andcarbo-nitrided Pyrowear® 675.

FIG. 8 is a schematic drawing of a rolling contact fatigue testingapparatus.

FIG. 9 shows a rolling contact fatigue life comparison betweencarburized and carbo-nitrided Pyrowear® 675.

FIG. 10 shows a SEM Micrograph of carbo-nitrided Pyrowear® 675.

DETAILED DESCRIPTION OF THE INVENTION

Although the following description primarily concerns Pyrowear® 675(hereinafter “P675”) samples that were carbo-nitrided using differentprocess parameters based on application requirements, the presentteachings are also applicable to other martensitic stainless steelshaving similar elemental compositions, such as e.g., CSS-42L of LatrobeSpecialty Steel Company, Latrobe, Pa., U.S.A.

The initial carburizing cycle was conducted in a temperature range of1625° F.-1680° F. (885° C.-916° C.) followed by a carbo-nitriding cyclein the temperature range of 1575° F.-1625° F. (857° C.-885° C.). Theheat treat cycle time can be between 15-18 hours to produce a hardcarburized case up to a depth of 900-1125 μm (38-45 mil).Saturation/diffusion cycles were alternately and repeatedly performed,in which acetylene was introduced into a vacuum chamber containing thesteel and then a vacuum is drawn to about 0.1 atm (about 10 kPa). Duringthe carbo-nitriding cycle, ammonia was simultaneously introducedtogether with the acetylene, followed again by the evacuation/diffusionphase. During the low pressure phases, carbon (and nitrogen, if present)contacting the surface of the metal diffuses into the surface (case) ofthe metal. The respective soaking temperatures were held constant duringboth phases.

The carbo-nitrided stainless steel may then be quenched in liquidnitrogen (about −196° C.), followed by tempering twice at 600° F. (315°C.) for 2 hours. The resulting carbo-nitrided stainless steel had aVickers hardness of about 840 HV (65.0 HRc).

In the carbo-nitriding step, the ammonia-to-carbon differential ratiocan preferably be in the range of 6-8:1. In such an embodiment, thehardened case of the article exhibits a presence of nitrogen up to adepth of 250 μm (10 mil). Approximately 0.5 wt % nitrogen was detectedat the surface and 0.2 wt % nitrogen was detected to the depth of about250 μm (10 mil). The process resulted in a carbon content in thehardened case in the range of 1.5-2 wt %. X-ray diffraction (XRD)results showed the formation of different metal nitrides (M_(x)N & MN,where M represents metal) not present in carburized P675. The processformed Fe_(x)N, Cr_(x)N, V_(x)N, as will be discussed further below.

The heat-treatment process according to the present teachings resultedin finely distributed carbides without carbide segregation at grainboundaries, as shown in FIG. 10. A significant improvement in hardnessprofile, hot hardness and residual stress profiles were also observed,as will be discussed below. Furthermore, anodic polarization corrosiontests showed significant improvement in corrosion performance ofcarbo-nitrided (C-N) Pyrowear® 675. For example, the carbo-nitrided P675exhibited at least 50% lower corrosion rate/yr as compared to acarburized P675 sample.

Further characterizing data is provided in the following, in which aP675 sample that was carbo-nitrided according to the above-mentionedprocess conditions (hereinafter “the carbo-nitrided P675 specimen”) iscompared to a P675 sample that was only carburized and thecarbo-nitriding step was omitted (hereinafter “the carburized P675specimen”).

Hardness Depth Profile

The carburized and carbo-nitrided P675 specimens were cross sectionedand polished for hardness measurements. Micro-hardness at roomtemperature was measured as a function of depth (case to core). Threehardness measurements were taken at each depth and average hardness arereported. The hardness profiles for carbo-nitrided (C-N) samples areshown in FIG. 1. A significant increase in hardness was observed withthe carbo-nitrided P675 specimen in comparison with the carburized P675specimen. While not wishing to be bound by theory, the substantialimprovement in hardness of C-N material over the carburized P675specimen is believed to be a significant factor in improving fatiguelife and reducing wear rate, as will be demonstrated below.

Hot Hardness Measurement

Hot hardness measurements for the carburized and carbo-nitrided P675specimens were performed at room temperature, 149° C. (300° F.), 204° C.(400° F.), 260° C. (500° F.) and 315° C. (600° F.). Five hardnessmeasurements were taken at each temperature and average hardness arereported. Hardness was measured using Rockwell A scale and thenconverted to Rockwell C scale. The hardness of the carbo-nitrided P675specimen is compared to the carburized P675 specimen in FIG. 2. Thecarbo-nitrided P675 specimen exhibited a hot hardness profile superiorto the carburized P675 specimen. While not wishing to be bound bytheory, the substantial improvement in hot harness of carbo-nitridedmaterial over carburized materials is believed to be a significantfactor in improving fatigue life and allowing higher operatingtemperatures, as will be demonstrated below.

Corrosion Test

An anodic polarization test was used to quantify the corrosionresistance of the carburized and carbo-nitrided P675 specimens. Thetests were conducted in 5% NaCl solution. When steel, particularly steelhaving a high Cr content, i.e. corrosion resistant steel, is exposed toa chloride-containing medium, it experiences a localized corrosion, i.e.corrosion pitting, rather than uniform corrosion. To determine thecorrosion resistance of a material, the voltage at which the materialbegins to pit, i.e. the pitting potential (Epit), is determined from thepolarization curve. The more positive (higher) the pitting potential,the more resistant the material is to pitting corrosion.

The experimental test conditions and parameters were as follows. Allsamples were prepared by polishing to 600 grit (SiC paper), then rinsingwith water, acetone, methanol, DI water and finally air drying. Afterdrying, the samples were allowed to sit for at least 2 hours prior totesting.

Anodic potentiodynamic scans were performed using a testing solution ofsimulated seawater (0.8 M NaCl). The open circuit potential (OCP) wasmonitored for 10 minutes prior to performing the scan to ensurestability of the corrosion potential. Polarization scan limits wereperformed from −0.1 (V) vs. OCP to +0.25 (V) vs. OCP. The scan rate was0.5 mV/sec. The reference electrode used in all tests was saturated KCl.

Bearing steel samples were tested by mounting each sample on the side ofan electrochemical cell so that only the polished face was exposed tothe testing solution. The working electrode (bearing steel sample) wasmounted on the face of the corrosion cell and experiments were performedusing a calomel reference electrode (SCE) and an expanded platinum meshcounter electrode. The electrical connection to the working electrodewas made by spot welding pure nickel wire to the edge of the sample. Thecorrosion rate was calculated from a Tafel fit software.

As shown in FIG. 3, the carbo-nitrided P675 specimen exhibited acorrosion rate that was an order of magnitude lower than the twocarburized P675 specimens (tempered respectively at 932° F. and 600°F.).

Corrosion is one of the main causes of bearing and gear failure. Use ofhighly corrosion resistant bearing material has the potential tosignificantly improve safety and reliability of advanced turbineengines. Improved bearing and gear material treated with the processaccording to the present teachings may also be advantageously utilizedin a variety of high performance military and commercial aircraftturbine engines. Commercial and military ground vehicles also maybenefit from the present teachings, as the present process is capable ofimproving corrosion resistance and thus system reliability and safety.

X-Ray Diffraction Analysis

The carburized and carbo-nitrided P675 specimens were analyzed by XRDtechnique to determine phases formed by each process. Introduction ofnitrogen by the carbo-nitriding (C-N) process resulted in formation ofadditional metal nitrides (MN & M_(x)N, wherein M represents metal) inthe hard case as shown in FIG. 4. While not wishing to be bound bytheory, the formation of these metal nitrides is believed to beimportant to improving corrosion resistance.

Metal nitrides included in the case of the carbo-nitrided steel specimeninclude one or more of V₂N, Cr₂N, VN, Fe₃N and Fe₃N₄.

Carbon-Nitrogen Determination

Auger electron spectroscopy (AES) was used to determine the weightpercentages of nitrogen and carbon. The specimens were cross-sectionedand polished for AES analysis. After polishing, the specimens wereremoved from their mount and rinsed with acetone prior to analysis. Thespecimens were loaded into the AES vacuum system along with a polished1074 steel sample and a sample of 99.9% iron foil, which were used asreferences. The AES vacuum system was baked overnight to achieve a basepressure in the 10⁻¹⁰ Torr range. This is necessary in order to attain avery low background carbon level. This work was done using a Varianmodel 981-2707 Auger spectrometer. The electron beam diameter for thiswork was ˜20 μm.

Before initiating the profiles, the samples were argon ion-sputtered forseveral minutes in order to remove residual surface contaminants. Theion gun was left on during the profiles in order to avoid anyre-contamination. A carbon level of 1.09±0.04 wt % was measured on the1074 steel sample, as compared with the expected composition range of0.70-0.80 wt %. For the iron foil, the carbon level was measured to be<0.2 wt %. Nitrogen in the range of 0.2 to 0.6 wt % was detected up tothe depth of ˜250 μm (10 mil) as shown by AES nitrogen profile in FIG.5.

Residual Stress

X-ray diffraction residual stress measurements were made at the surfaceand at nominal depths of 0.5, 1.0, 2.0, 3.0, 5.0, 7.0 and 10.0×10⁻³ in.(13, 25, 51, 76, 127, 178 and 254×10⁻³ mm). Measurements were made inthe radial direction at the center of the flat face.

X-ray diffraction residual stress measurements were performed using atwo-angle sine-squared-psi technique, in accordance with SAE HS-784,employing the diffraction of chromium K-alpha radiation from the (211)planes of the BCC structure of the Pyrowear® 675. The diffraction peakangular positions at each of the psi tilts employed for measurement weredetermined from the position of the K-alpha 1 diffraction peak separatedfrom the superimposed K-alpha doublet assuming a Pearson VII functiondiffraction peak profile in the high back-reflection region. Thediffracted intensity, peak breadth, and position of the K-alpha 1diffraction peak were determined by fitting the Pearson VII functionpeak profile using a least squares regression after correction for theLorentz polarization and absorption effects and for a linearly slopingbackground intensity. Material was removed electrolytically forsubsurface measurements in order to minimize possible alteration of thesubsurface residual stress distribution as a result of material removal.

All data obtained as a function of depth were corrected for the effectsof the penetration of the radiation employed for residual stressmeasurement into the subsurface stress gradient. The stress gradientcorrection applied to the last depth measured is based upon anextrapolation to greater depths and may result in over correction at thelast depth if the stress profile has been terminated in the presence ofa steep gradient. Corrections for sectioning stress relaxation and forstress relaxation caused by layer removal are applied as appropriate.

As was noted above, X-ray diffraction residual stress measurements weremade at the surface and at nominal depths of 0.5, 1.0, 2.0, 3.0, 5.0,7.0 and 10.0×10⁻³ in. (13, 25, 51, 76, 127, 178 and 254×10⁻³ mm). X-raydiffraction technique was used to measure residual stress and retainedaustenite as a function of depth. As shown in FIG. 6, thecarbo-nitriding process according to the present teachings producedhigher compressive stresses, which are desirable to improve fatiguelife.

Retained Austenite Measurements

The mean volume percent of retained austenite (RA) calculated from thefour independently determined diffraction peak integrated intensityratios and the standard deviation about the mean value are presented inFIG. 7. Uncertainties in austenite measurement that exceed one percentmay be the result of preferred orientation present in a sample. In allx-ray diffraction quantitative analysis methods, the crystals in thesample are assumed to be randomly oriented so that the diffraction peakintegrated intensities vary only with the relative volume fractions ofthe phases present. Standard deviations on the order of one volumepercent are attainable on samples that have truly random orientation.However, because the relative intensities of the diffraction peaks foreach phase depend upon the crystal structure and the preferredorientation (texture) present, the variation between the individualaustenite results reported is not actually random experimental error.

Previous studies comparing the results obtained on orthogonal faces ofhighly textured steels have shown that the mean austenite contentcalculated is generally more reliable than indicated by the “standarddeviation” reported. Carbide peaks near the austenite and martensitepeaks may contribute to the integrated intensity and cause additionalerror and are avoided whenever possible. Performance of the technique ismonitored using a secondary reference standard which is certified tocontain 25.4% austenite with reference to NIST Standard ReferenceMaterial No. 487.

A measurement on this sample during the investigation produced a valueof 25.0±0.5 percent. The RA profiles for the carburized andcarbo-nitrided P675 specimens are shown in FIG. 7.

Rolling Contact Fatigue Testing

Rolling contact fatigue (RCF) tests were conducted using a modified RCFtester. A schematic of the tester is shown in FIG. 8. The “ball-on-rod”tester consists of a rotating cylindrical test specimen 1 (9.525 mm indiameter) stressed by rolling contact with three radially loaded balls 2(12.7 mm in diameter) held spaced apart in a retainer 3. The balls 2 areloaded against the rod 1 by two stationary tapered races 4 at apredetermined calibrated thrust load applied by three compressionsprings 5. To accelerate failure, high Hertzian stresses were used. Thetest configuration was set such that pure rolling conditions existbetween the rod and balls. A constant rolling speed of 1.8 m/s (3600rpm) was used for all experiments.

In the present configuration, the test head is heated using band heaters6. The test specimens 1 were continuously lubricated by dripping roomtemperature test lubricant onto the end of the specimen 1. Anaccelerometer, placed on the test head, was coupled with a shutdowndevice and monitored the vibrations of the assembly. When a presetvibration level was exceeded, thereby indicating the presence of fatiguespall or a surface crack, the motor, band heaters, and oil supply wereautomatically stopped. The test duration was recorded by an hour meterconnected electronically to the motor. The test time required for afatigue spall to develop on the rod was recorded, and the numbers ofstress cycles were later calculated. Twenty tests were run on each testrod 1 at different axial positions for each lubricant/bearing material.New balls 2 were used for each test, and the races 4 were replaced after20 tests.

The races 4 were made from VIM-VAR M50. Silicon nitride (Toshiba,TSN-03NH, Grade 5) rolling elements 2 were used to simulate a hybridbearing configuration. Cages 3 were fabricated from AISI 4340.

A fully formulated polyolester base lubricant conforming toMIL-PRF-23699 (BP2197) specification was used as the lubricant withviscosity of 5 cSt @ 100° C. The calculated specific film thickness (X)of values between 0.17-0.25 suggested that all tests were performedunder boundary lubrication. The test specimens 1 were continuouslylubricated at a constant flow rate of 22 ml/hr. The test lubricant wasat room temperature prior to flowing into the test section.

To accelerate fatigue failure, a maximum Hertzian stress of 5.5 GPa (800ksi) was used. The test was performed at 350° F. (177° C.). Asurface-initiated fatigue spall on the rod 1 was the test failingcriterion. Tests were suspended after 300 hours of testing time, if nofatigue spall initiation occurred on the rod 1. The RCF results wereanalyzed using a two-parameter Weibull distribution with suspensions.Failures of the M50 race and the Si₃Ni₄ ball were treated assuspensions. Also, the tests with a run time in excess of 300 hourswithout a fatigue spall were also treated as suspensions in the Weibullanalysis.

As shown in FIG. 9, the carbo-nitrided P675 specimen showedstatistically significant improved fatigue life compared to thecarburized P675 specimen (L₁₀ life improved by 122% and L₅₀ lifeimproved by 146%). While not wishing to be bound by theory, significantimprovement in fatigue life will result in improved reliability, longercomponent life and reduction in maintenance costs.

Microstructure

The microstructure of the carbo-nitrided P675 specimen is shown in FIG.10, in which fine, evenly-distributed carbides in the case can be seen.

Comparison with Other Steel Treatment Processes

An overall comparison of various steel treatment processes, as comparedto carbo-nitriding according to the present teachings (bottommost entry)are shown below in Table 1. As can be seen, only the presentcarbo-nitriding technique was capable of providing both superiorcorrosion resistance and superior fatigue life performance.

TABLE 1 Specific Surface Corrosion Fatigue Life Treatment resistancePerformance Carburized poor Accepted (used as base line) Cr ionimplantation* poor Not tested TiN coating* good poor TiN + Ag coating*poor poor low plastic burnishing* poor acceptable laser shock penning*poor not tested Carbo-nitrided superior superior *Post carburizationtreatment

Representative, non-limiting examples of the present invention weredescribed above in detail. This detailed description is merely intendedto teach a person of skill in the art further details for practicingpreferred aspects of the present teachings and is not intended to limitthe scope of the invention. Furthermore, each of the additional featuresand teachings disclosed above may be utilized separately or inconjunction with other features and teachings to provide improvedstainless steel articles and methods for manufacturing the same.

Moreover, combinations of features and steps disclosed in the abovedetail description may not be necessary to practice the invention in thebroadest sense, and are instead taught merely to particularly describerepresentative examples of the invention. Furthermore, various featuresof the above-described representative examples, as well as the variousindependent and dependent claims below, may be combined in ways that arenot specifically and explicitly enumerated in order to provideadditional useful embodiments of the present teachings.

All features disclosed in the description and/or the claims are intendedto be disclosed separately and independently from each other for thepurpose of original written disclosure, as well as for the purpose ofrestricting the claimed subject matter, independent of the compositionsof the features in the embodiments and/or the claims. In addition, allvalue ranges or indications of groups of entities are intended todisclose every possible intermediate value or intermediate entity forthe purpose of original written disclosure, as well as for the purposeof restricting the claimed subject matter.

1. A method comprising: (a) providing an article comprised, at least inpart, of a martensitic stainless steel, (b) carburizing the articlewithin a temperature range of 1625° F.-1680° F. (885° C.-916° C.), and(c) then carbo-nitriding the article within a temperature range of 1575°F.-1625° F. (857° C.-885° C.), thereby producing a case-hardenedmartensitic stainless steel article.
 2. The method according to claim 1,wherein the stainless steel in step (a) consists essentially of0.02-0.50 wt % C, 0.1-1.5 wt % Mn, 0.10-2.0 wt % Si, 8.0-20.0 wt % Cr,1.0-3.5 wt % Ni, 0.40-3.0 wt % Mo, 0.40-2.0 wt % V, 1.0-10.0 wt % Co,the balance being Fe.
 3. The method according to claim 2, whereinstainless steel in step (a) consists essentially of 0.03-0.10 wt % C,0.5-1.0 wt % Mn, 0.20-0.6 wt % Si, 11.0-15.0 wt % Cr, 2.40-3.0 wt % Ni,1.50-2.0 wt % Mo, 0.40-0.80 wt % V, 5.0-6.0 wt % Co, the balance beingFe.
 4. The method according to claim 2, wherein the stainless steel instep (a) contains the following weight percentages of elements: Mo+V=0.8to 4.0 wt % and Co+Ni=2.0 to 12.0 wt %.
 5. (canceled)
 6. The methodaccording to claim 1, wherein a nitrogen:carbon differential ratio instep (c) is between about 6-8:1.
 7. The method according to claim 6,wherein nitrogen is provided by ammonia and carbon is provided byacetylene and/or propane.
 8. The method according to claim 1, whereinthe total time for steps (b) and (c) is between 15-18 hours. 9.-12.(canceled)
 13. The method according to claim 1, wherein thecase-hardened martensitic stainless steel article is subsequently: (d)quenched in liquid nitrogen and (e) then tempered twice at about 575°F.-625° F. (301° C.-329° C.) for 2 hours. 14.-21. (canceled)
 22. Themethod according to claim 1, wherein the article is a bearing ring.23.-39. (canceled)
 40. A bearing ring comprised of a carbo-nitridedmartensitic stainless steel having: a hard carburized case up to a depthof about 900-1125 μm (38-45 mil), a presence of nitrogen or nitrides upto depth of 250 μm (10 mil), about 0.3-0.7 wt % nitrogen at the surfaceand about 0.1-0.3 wt % nitrogen at a depth of about 250 μm (10 mil), andcarbon in the range of 1.5-2 wt % in the hardened case.
 41. The bearingring according to claim 40, wherein the bearing ring has a coreconsisting essentially of 0.02-0.50 wt % C, 0.1-1.5 wt % Mn, 0.10-2.0 wt% Si, 8.0-20.0 wt % Cr, 1.0-3.5 wt % Ni, 0.40-3.0 wt % Mo, 0.40-2.0 wt %V, 1.0-10.0 wt % Co, the balance being Fe.
 42. The bearing ringaccording to claim 41, wherein the bearing ring has a core consistingessentially of 0.03-0.10 wt % C, 0.5-1.0 wt % Mn, 0.20-0.6 wt % Si,11.0-15.0 wt % Cr, 2.40-3.0 wt % Ni, 1.50-2.0 wt % Mo, 0.40-0.80 wt % V,5.0-6.0 wt % Co, the balance being Fe.
 43. The bearing ring according toclaim 41, wherein the bearing ring has a core containing the followingweight percentages of elements: Mo+V=0.8 to 4.0 wt % and Co+Ni=2.0 to12.0 wt %.
 44. The bearing ring according to claim 40, wherein thebearing ring has a core consisting of one of (in wt %): (a) 0.07 C, 0.65Mn, 0.40 Si, 13.00 Cr, 2.60 Ni, 1.80 Mo, 0.60 V, 5.40 Co, the balancebeing Fe, and (b) 0.12 C, 14.00 Cr, 0.60 V, 2.00 Ni, 4.75 Mo, 12.5 Co,0.02 Cb, the balance being Fe.
 45. The bearing ring according to claim40, wherein the hardened case of the bearing ring has a hardness of atleast 800 Hv (˜63.7 HRc) at a depth between 0.010 to 0.039 in (0.25 to1.0 mm).
 46. The bearing ring according to claim 40, wherein thehardened case of the bearing ring has a hardness of between 800-850 Hv(˜63.7 to 65.4 HRc) at a depth between 0.010 to 0.039 in (0.25 to 1.0mm).
 47. The bearing ring according to claim 40, wherein the hardenedcase of the bearing ring has a hardness of at least 63 HRc (˜772 HV) at300° F. (149° C.). 48.-50. (canceled)
 51. The bearing ring according toclaim 42, wherein the hardened case of the bearing ring has: a hardnessof between 800-850 Hv (˜63.7 to 65.4 HRc) at a depth between 0.010 to0.039 in (0.25 to 1.0 mm), a hardness of at least 63 HRc (˜772 HV) at149° C. (300° F.), a hardness of at least 62 HRc (˜745 HV) at 204° C.(400° F.) and a residual stress of at least 500 MPa.
 52. The bearingring according to claim 51, wherein at least one portion of the hardenedcase of the bearing ring comprises at least 16 volume percent ofretained austenite.
 53. A method comprising: (a) placing an article intoa vacuum chamber, the article being comprised of a martensitic stainlesssteel consisting essentially of 0.02-0.50 wt % C, 0.1-1.5 wt % Mn,0.10-2.0 wt % Si, 8.0-20.0 wt % Cr, 1.0-3.5 wt % Ni, 0.40-3.0 wt % Mo,0.40-2.0 wt % V, 1.0-10.0 wt % Co, the balance being Fe, (b) vacuumcarburizing the article within a temperature range of 1625° F.-1680° F.(885° C.-916° C.) by repeating a plurality of cycles comprising (i)introducing acetylene into the vacuum chamber and (ii) then evacuatingthe vacuum chamber to about 0.1 atm (about 10 kPa), and (c) then vacuumcarbo-nitriding the article within a temperature range of 1575° F.-1625°F. (857° C.-885° C.) by repeating a plurality of cycles comprising (i)introducing acetylene and ammonia into the vacuum chamber and (ii) thenevacuating the vacuum chamber to about 0.1 atm (about 10 kPa).